An improved structure of water mill has a body provided with a handle part, a body part, a working head base and a pressing control switch. The handle part is provided with an intake passage, an exhaust passage, and a water inlet reversing passage. By triggering the pressing control switch, a high-pressure gas is guided into the intake passage and the body to drive the water grinder. A water pump gas outlet is provided with a double sleeve tube connected to a sealed water storage tank. The discharged high-pressure gas is guided into the sealed water storage tank through the outer tube of the double sleeve tube, so that the stored water is pressed back to the double sleeve tube and is simultaneously guided into the water inlet reversing passage. Finally, the water is guided out to the working head base, with simultaneous grinding and water cooling.